This invention relates to molding resins. In one aspect, the invention relates to molding resins that upon cure are transparent to ultraviolet light or radiation while in another aspect, the invention relates to molds made from these resins. In yet another aspect, the invention relates to using the UV-transparent molds to shape fiber preforms in which a UV-curable composition is used as the preform binder.
The process of making fiber preforms is greatly enhanced by using UV-curable binders rather than heat-curable binders. The former generally operates at ambient temperature and generates only a small amount of heat. The latter, by its very nature, requires elevated temperatures, often in excess of 300.degree. C., and thus generates a significant amount of heat. As such, processes using heat-curable binders generally generate significantly greater quantities of volatile organic compound emissions, consume significantly greater amounts of energy, and require significantly longer periods of time to rigidize the preform, than processes using UV-curable binders.
However, processes using UV-curable binders generally require a UV-transparent mold or forming tool to hold the fiber reinforcement in place while the binder is curing. As here used, "UV-transparent" means that the mold or forming tool transmits or conducts sufficient UV radiation or light to rapidly cure the UV-curable binder. In addition to being UV-transparent, important properties of the material used in construction of the mold are formability and strength. The materials are desirably easily shaped into the geometry of the mold, and are sufficiently strong to withstand the pressures required to shape the fibers placed against their surface. Furthermore, the mold should withstand the heat generated by the source of the UV light, e.g. a UV lamp (many UV lamps also generate infrared radiation, and thus heat), and should be resistant to photodegradation from exposure to the UV radiation used to cure the preform binder.
While many common glasses are clear, and thermoformable plastics have some of the desired properties, each of these materials is lacking in certain essential characteristics. For example, glass efficiently transmits UV radiation in the range of 330 nm to 400 nm, but fractures easily and is very difficult to form. Some thermoplastic materials, such as polycarbonate and polystyrene, can be easily formed but absorb quite strongly in the 330 to 400 nm range. Some acrylic thermoplastics have good UV transmission properties but are difficult to form, while other grades form quite well but absorb strongly any UV radiation below 370 nm. While many thermoplastic materials can be machined shaped, this process is time consuming and results in a large amount of material waste. Furthermore, many thermoplastic materials distort from the heat generated by UV lamps, or degrade upon exposure to UV radiation.